1. Field of the Invention
This invention relates to the production of very high molecular weight homopolymers from acrylamide monomer by the process of aqueous polymerization.
2. Description of the Prior Art
In U.S. Pat. No. 3,573,263 there is disclosed the preparation of water-soluble, high molecular weight synthetic polymers from monomers such as acrylic acid and acrylamide by the use of a redox system in conjunction with the use of an azo compound free-radical source. As indicated therein, the use of a redox catalyst only, without the additional azo compound catalyst, results in an exotherm during the process which causes greater initiator activity and thus, results in a polymer having a low molecular weight.
The problems involved in the preparation of high molecular weight polymers utilizing solution or emulsion polymerization of ethylenically unsaturated monomers are extensively discussed in U.S. Pat. No. 4,103,080, incorporated herein by reference. In prior art solution polymerization processes, a polymerization temperature of about 60 to about 70.degree. C. has been used which causes the polymerization initiator to decompose rapidly. This results in too many polymer chains starting to form simultaneously with the end effect that too many short chains are produced. The resulting polymers are inadequate since they are low in molecular weight. This has led to the use of such slower-decomposing polymerization redox initiators as amines and ammonia. With the use of ammonia, generally extensive pH controls must be utilized to avoid the loss of ammonia and an uncontrolled course of reaction.
The process of U.S. Pat. No. 4,103,080 involves the addition of catalysts in at least three steps at intervals during the course of the polymerization in order to maintain the initiator concentration low enough so as to obtain the very high molecular weight polymers desired. In U.S. Pat. No. 4,042,772, the problem of obtaining acrylamide and acrylamide-acrylic acid polymers utilizing contaminated acrylamide is solved by the use of urea as an additive during the polymerization. Use of a water-in-oil emulsion polymerization process is described as particularly suitable for the production of said polymers utilizing a redox polymerization catalyst. Bikales in Water-Soluble Polymers, Plenum Press, 1973, page 215, describe a commercial process for the production of acrylamide polymers in which one of the components of the redox catalyst system is added gradually to a second component of the redox system. For instance, the reducing agent is added gradually to control the rate of polymerization and prevent the temperature from becomming excessive. Other processes describing the polymerization of ethylenically unsaturated monomers utilizing redox polymerization catalysts are U.S. Pat. No. 3,509,113 and U.S. Pat. No. 4,020,256. There is no indication in the prior art that very high molecular weight polymers can be obtained by a process in which a minimum of a redox catalyst, i.e., just sufficient to initiate polymerization, is utilized. In the process disclosed herein, said minimum is determined, for instance, by titrating a peroxy compound into a reaction mixture containing one member of a redox pair as an activator for the peroxy compound.